Empirical Models of Pollen Limitation, Resource Acquisition, and Mast Seeding by a Bee‐Pollinated Wildflower

Oak masting drivers vary between populations depending on their climatic environments

Large interannual variation in seed production, called masting, is very common in wind-pollinated tree populations and has profound implications for the dynamics of forest ecosystems and the epidemiology of certain human diseases.^1,2,3,4,5 Comparing the reproductive characteristics of populations established in climatically contrasting environments would provide powerful insight into masting mechanisms, but the required data are extremely scarce. We built a database from an unprecedented fine-scale 8-year survey of 150 sessile oak trees (Quercus petraea) from 15 populations distributed over a broad climatic gradient, including individual recordings of annual flowering effort, fruiting rate, and fruit production. Although oak masting was previously considered to depend mainly on fruiting rate variations,^6,7 we show that the female flowering effort is highly variable from year to year and explains most of the fruiting dynamics in two-thirds of the populations. What drives masting was found to differ among populations living under various climates. In soft-climate populations, the fruiting rate increases initially strongly with the flowering effort, and the intensity of masting results mainly from the flowering synchrony level between individuals. By contrast, the fruiting rate of harsh-climate populations depends mainly on spring weather, which ensures intense masting regardless of the flowering synchronization level. Our work highlights the need for jointly measuring flowering effort and fruit production to decipher the diversity of masting mechanisms among populations. Accounting for such diversity will be decisive in proposing accurate, and possibly contrasted, scenarios about future reproductive patterns of perennial plants with ongoing climate change and their numerous cascading effects.

Flowering synchrony drives reproductive success in a wind-pollinated tree

Synchronised and quasi-periodic production of seeds by plant populations, known as masting, is implicated in many ecological processes, but how it arises remains poorly understood. Flowering and pollination dynamics are hypothesised to provide the mechanistic link for the observed relationship between weather and population-level seed production. We report the first experimental test of the phenological synchrony hypotheses as a driver of pollen limitation in mast seeding oaks (Quercus ilex). Higher flowering synchrony yielded greater pollination efficiency, which resulted in 2-fold greater seed set in highly synchronised oaks compared to asynchronous individuals. Pollen addition removed the negative effect of asynchronous flowering on seed set. Because phenological synchrony operates through environmental variation, this result suggests that oak masting is synchronised by exogenous rather than endogenous factors. It also points to a mechanism by which changes in flowering phenology can affect plant reproduction of mast-seeding plants, with subsequent implications for community dynamics.

Environmental Veto Synchronizes Mast Seeding in Four Contrasting Tree Species

Synchronized and variable reproduction by perennial plants, called mast seeding, is a major reproductive strategy of trees. The need to accumulate sufficient resources after depletion following fruiting (resource budget), the efficiency of mass flowering for outcross pollination (pollen coupling), or the external factors preventing reproduction (environmental veto) could all synchronize masting. We used seed production data for four species (Quercus ilex, Quercus humilis, Sorbus aucuparia, and Pinus albicaulis) to parametrize resource budget models of masting. Based on species life-history characteristics, we hypothesized that pollen coupling should synchronize reproduction in S. aucuparia and P. albicaulis, while in Q. ilex and Q. humilis, environmental veto should be a major factor. Pollen coupling was stronger in S. aucuparia and P. albicaulis than in oaks, while veto was more frequent in the latter. Yet in all species, costs of reproduction were too small to impose a replenishment period. A synchronous environmental veto, in the presence of environmental stochasticity, was sufficient to produce observed variability and synchrony in reproduction. In the past, vetoes like frost events that prevent reproduction have been perceived as negative for plants. In fact, they could be selectively favored as a way to create mast seeding.

Correlated seed failure as an environmental veto to synchronize reproduction of masting plants

Variable, synchronized seed production, called masting, is a widespread reproductive strategy in plants. Resource dynamics, pollination success, and, as described here, environmental veto are possible proximate mechanisms driving masting. We explored the environmental veto hypothesis, which assumes that reproductive synchrony is driven by external factors preventing reproduction in some years, by extending the resource budget model of masting with correlated reproductive failure. We ran this model across its parameter space to explore how key parameters interact to drive seeding dynamics. Next, we parameterized the model based on 16 yr of seed production data for populations of red (Quercus rubra) and white (Quercus alba) oaks. We used these empirical models to simulate seeding dynamics, and compared simulated time series with patterns observed in the field. Simulations showed that resource dynamics and reproduction failure can produce masting even in the absence of pollen coupling. In concordance with this, in both oaks, among-year variation in resource gain and correlated reproductive failure were necessary and sufficient to reproduce masting, whereas pollen coupling, although present, was not necessary. Reproductive failure caused by environmental veto may drive large-scale synchronization without density-dependent pollen limitation. Reproduction-inhibiting weather events are prevalent in ecosystems, making described mechanisms likely to operate in many systems.

The Moran effect and environmental vetoes: phenological synchrony and drought drive seed production in a Mediterranean oak

Masting is the highly variable production of synchronized seed crops, and is a common reproductive strategy in plants. Weather has long been recognized as centrally involved in driving seed production in masting plants. However, the theory behind mechanisms connecting weather and seeding variation has only recently been developed, and still lacks empirical evaluation. We used 12-year long seed production data for 255 holm oaks (Quercus ilex), as well as airborne pollen and meteorological data, and tested whether masting is driven by environmental constraints: phenological synchrony and associated pollination efficiency, and drought-related acorn abscission. We found that warm springs resulted in short pollen seasons, and length of the pollen seasons was negatively related to acorn production, supporting the phenological synchrony hypothesis. Furthermore, the relationship between phenological synchrony and acorn production was modulated by spring drought, and effects of environmental vetoes on seed production were dependent on last year's environmental constraint, implying passive resource storage. Both vetoes affected among-tree synchrony in seed production. Finally, precipitation preceding acorn maturation was positively related to seed production, mitigating apparent resource depletion following high crop production in the previous year. These results provide new insights into mechanisms beyond widely reported weather and seed production correlations.

Time-lagged effects of weather on plant demography: drought and Astragalus scaphoides

Temperature and precipitation determine the conditions where plant species can occur. Despite their significance, to date, surprisingly few demographic field studies have considered the effects of abiotic drivers. This is problematic because anticipating the effect of global climate change on plant population viability requires understanding how weather variables affect population dynamics. One possible reason for omitting the effect of weather variables in demographic studies is the difficulty in detecting tight associations between vital rates and environmental drivers. In this paper, we applied Functional Linear Models (FLMs) to long-term demographic data of the perennial wildflower, Astragalus scaphoides, and explored sensitivity of the results to reduced amounts of data. We compared models of the effect of average temperature, total precipitation, or an integrated measure of drought intensity (standardized precipitation evapotranspiration index, SPEI), on plant vital rates. We found that transitions to flowering and recruitment in year t were highest if winter/spring of year t was wet (positive effect of SPEI). Counterintuitively, if the preceding spring of year t - 1 was wet, flowering probabilities were decreased (negative effect of SPEI). Survival of vegetative plants from t - 1 to t was also negatively affected by wet weather in the spring of year t - 1 and, for large plants, even wet weather in the spring of t - 2 had a negative effect. We assessed the integrated effect of all vital rates on life history performance by fitting FLMs to the asymptotic growth rate, log(λt). Log(λt) was highest if dry conditions in year t - 1 were followed by wet conditions in the year t. Overall, the positive effects of wet years exceeded their negative effects, suggesting that increasing frequency of drought conditions would reduce population viability of A. scaphoides. The drought signal weakened when reducing the number of monitoring years. Substituting space for time did not recover the weather signal, probably because the weather variables varied little between sites. We detected the SPEI signal when the analysis included data from two sites monitored over 20 yr (2 × 20 observations), but not when analyzing data from four sites monitored over 10 yr (4 × 10 observations).

Masting in wind-pollinated trees: system-specific roles of weather and pollination dynamics in driving seed production

Masting, the highly variable production of synchronized large seed crops, is a common reproductive strategy in plant populations. In wind-pollinated trees, flowering and pollination dynamics are hypothesized to provide the mechanistic link for the well-known relationship between weather and population-level seed production. Several hypotheses make predictions about the effect of weather on annual pollination success. The pollen coupling hypothesis predicts that weather and plant resources drive the flowering effort of trees, which directly translates into the size of seed crops through efficient pollination. In contrast, the pollination Moran effect hypothesis predicts that weather affects pollination efficiency, leading to occasional bumper crops. Furthermore, the recently formulated phenology synchrony hypothesis predicts that Moran effects can arise because of weather effects on flowering synchrony, which, in turn, drives pollination efficiency. We investigated the relationship between weather, airborne pollen, and seed production in common European trees, two oak species (Quercus petraea and Q. robur) and beech (Fagus sylvatica) with a 19-yr data set from three sites in Poland. Our results show that warm summers preceding flowering correlated with high pollen abundance and warm springs resulted in short pollen seasons (i.e., high flowering synchrony) for all three species. Pollen abundance was the best predictor for seed crops in beech, as predicted under pollen coupling. In oaks, short pollen seasons, rather than pollen abundance, correlated with large seed crops, providing support for the pollination Moran effect and phenology synchrony hypotheses. Fundamentally different mechanisms may therefore drive masting in species of the family Fagacae.

Wild bee nutritional ecology: predicting pollinator population dynamics, movement, and services from floral resources

Pollination services are inherently shaped by floral resource availability, through the mediation of pollinator population dynamics and the influence on energetically costly processes, such as foraging. Here, we review recent insights that have improved our mechanistic understanding of how floral resources shape bee populations and pollination services. Our scope includes advances in our understanding of how individual bees and their populations are shaped by nutrient availability; investigations into how contemporary floral resource landscapes influence foraging; and new insights into how these relationships are indirectly impacted by biotic and abiotic factors across communities and landscapes. Throughout our review, we take a mechanistic, multi-scalar approach that highlights the complexity of interactions between floral resources and bees, across space and time.

Direct coupling: a possible strategy to control fruit production in alternate bearing

We investigated the theoretical possibility of applying phenomenon of synchronization of coupled nonlinear oscillators to control alternate bearing in citrus. The alternate bearing of fruit crops is a phenomenon in which a year of heavy yield is followed by an extremely light one. This phenomenon has been modeled previously by the resource budget model, which describes a typical nonlinear oscillator of the tent map type. We have demonstrated how direct coupling, which could be practically realized through grafting, contributes to the nonlinear dynamics of alternate bearing, especially phase synchronization. Our results show enhancement of out-of-phase synchronization in production, which depends on initial conditions obtained under the given system parameters. Based on these numerical experiments, we propose a new method to control alternate bearing, say in citrus, thereby enabling stable fruit production. The feasibility of validating the current results through field experimentation is also discussed.

Mechanisms of mast seeding: resources, weather, cues, and selection

546 I. 546 II. 547 III. 548 IV. 552 V. 554 VI. 556 VII. 558 VIII. 558 IX. 559 559 References 559 SUMMARY: Mast seeding is a widespread and widely studied phenomenon. However, the physiological mechanisms that mediate masting events and link them to weather and plant resources are still debated. Here, we explore how masting is affected by plant resource budgets, fruit maturation success, and hormonal coordination of cues including weather and resources. There is little empirical support for the commonly stated hypothesis that plants store carbohydrates over several years to expend in a high-seed year. Plants can switch carbohydrates away from growth in high-seed years, and seed crops are more probably limited by nitrogen or phosphorus. Resources are clearly involved in the proximate mechanisms driving masting, but resource budget (RB) models cannot create masting in the absence of selection because some underlying selective benefit is required to set the level of a 'full' seed crop at greater than the annual resource increment. Economies of scale (EOSs) provide the ultimate factor selecting for masting, but EOSs probably always interact with resources, which modify the relationship between weather cues and reproduction. Thus, RB and EOS models are not alternative explanations for masting - both are required. Experiments manipulating processes that affect mast seeding will help clarify the physiological mechanisms that underlie mast seeding.

Parameterisation and validation of a resource budget model for masting using spatiotemporal flowering data of individual trees

Synchronised and fluctuating reproduction by plant populations, called masting, is widespread in diverse taxonomic groups. Here, we propose a new method to explore the proximate mechanism of masting by combining spatiotemporal flowering data, biochemical analysis of resource allocation and mathematical modelling. Flowering data of 170 trees over 13 years showed the emergence of clustering with trees in a given cluster mutually synchronised in reproduction, which was successfully explained by resource budget models. Analysis of resources invested in the development of reproductive organs showed that parametric values used in the model are significantly different between nitrogen and carbon. Using a fully parameterised model, we showed that the observed flowering pattern is explained only when the interplay between nitrogen dynamics and climatic cues was considered. This result indicates that our approach successfully identified resource type-specific roles on masting and that the method is suitable for a wide range of plant species.

Resource limitation underlying multiple masting models makes mast seeding sensitive to future climate change

Mechanistic models can help resolve controversy over the responses of mast seeding plants to future environmental change. We evaluate drivers of mast seeding by: developing and validating a new mechanistic resource-based model of mast seeding using four 40-yr Chionochloa (snow tussock) datasets; and comparing the performance of competing empirically-based statistical models, that aim to approximate the mechanisms underlying mast seeding, in explaining simulated and observed data. Our mechanistic model explained 90-99% of the variation in Chionochloa flowering, with higher rates of stored resource mobilisation and lower probability of climatic induction of flowering occurring at lower fertility sites. Inter-annual variation in floral induction and the degree to which seeding is resource-limited explained shifts in the relative performance of different empirical models fitted to data simulated from the mechanistic model. Empirical models explicitly capturing the interaction between the floral induction cue and internal resource state underlying the resource-limited induction mechanism had > 8.7× the statistical support of alternatives when fitted to Chionochloa datasets. We find support for resource-limited floral induction with multiple empirical models consistent with this same mechanism. As both resource acquisition and flowering cues are climate sensitive, we expect climate change to impact upon patterns of mast seeding.

Understanding the alternate bearing phenomenon: Resource budget model

We consider here the resource budget model of plant energy resources, which characterizes the ecological alternate bearing phenomenon in fruit crops, in which high and low yields occur in alternate years. The resource budget model is a tent-type map, which we study in detail. An infinite number of chaotic bands are observed in this map, which are separated by periodic unstable fixed points. These m bands chaotic attractors become m/2 bands when the period-m unstable fixed points simultaneously collide with the chaotic bands. The distance between two sets of coexisting chaotic bands that are separated by a period-1 unstable fixed point is discussed. We explore the effects of varying a range of parameters of the model. The presented results explain the characteristic behavior of the alternate bearing estimated from the real field data. Effects of noise are also explored. The significance of these results to ecological perspectives of the alternate bearing phenomenon is highlighted.

Ability of matrix models to explain the past and predict the future of plant populations

Uncertainty associated with ecological forecasts has long been recognized, but forecast accuracy is rarely quantified. We evaluated how well data on 82 populations of 20 species of plants spanning 3 continents explained and predicted plant population dynamics. We parameterized stage-based matrix models with demographic data from individually marked plants and determined how well these models forecast population sizes observed at least 5 years into the future. Simple demographic models forecasted population dynamics poorly; only 40% of observed population sizes fell within our forecasts' 95% confidence limits. However, these models explained population dynamics during the years in which data were collected; observed changes in population size during the data-collection period were strongly positively correlated with population growth rate. Thus, these models are at least a sound way to quantify population status. Poor forecasts were not associated with the number of individual plants or years of data. We tested whether vital rates were density dependent and found both positive and negative density dependence. However, density dependence was not associated with forecast error. Forecast error was significantly associated with environmental differences between the data collection and forecast periods. To forecast population fates, more detailed models, such as those that project how environments are likely to change and how these changes will affect population dynamics, may be needed. Such detailed models are not always feasible. Thus, it may be wiser to make risk-averse decisions than to expect precise forecasts from models.

Why do sex ratio dimorphisms exist in Quercus masting? Evolution of imperfect synchronous reproduction in Monoecious trees

Masting is synchronous intermittent production of seeds in perennial plant populations. Some self-compatible monoecious Quercus species, such as oaks, exhibit sex ratio dimorphism and produce a certain proportion of male flowers, even in a year when no seed set occurs. To investigate sex ratio dimorphism in masting trees, we introduced sexual allocation as an evolutionary trait into the Resource Budget Model and examined the evolution of the sex ratio. Analytical and numerical findings show that (1) perfectly synchronous intermittent reproduction does not evolve; (2) if the fruiting cost of female flowers R(c) is sufficiently large and the pollen limitation beta is intermediate, annual reproduction does not evolve; (3) under conditions (2), sex ratio dimorphism can evolve across a wide region of parameter space; (4) after dimorphism is established, individuals with a female-biased sex ratio receive much more pollen supply from male-biased individuals and tend to show intermittent reproduction with or without synchrony; and (5) dimorphism is maintained with irregular and nearly discontinuous changes of sex ratio. These results suggest that sex ratio dimorphism contributes to improving pollen availability and causes resource depletion and the occurrence of intermittent reproduction in female-biased individuals.

Pollen and water limitation in Astragalus scaphoides, a plant that flowers in alternate years

Mast seeding is common in plant populations, but its causes have rarely been tested experimentally. We tested mechanisms of alternate-year flowering and fruit set in an iteroparous, bee-pollinated, herbaceous plant, Astragalus scaphoides, in semi-arid sagebrush steppe. Patterns of reproduction from 1986 to 1999 indicated that spring precipitation was a cue for synchronous flowering, and that increased pollination in high-flowering years was a fitness advantage of synchrony. We tested these patterns by adding supplemental water and pollen to plants in high- and low-flowering sites and years. Supplemental water had no effect on flowering or seed set, so water is not a proximate cue for reproduction, though it could be important over longer (>3 year) time scales. Supplemental pollination increased fruit set in low- but not high-flowering years, indicating that synchronous flowering increases pollination success. Many shorter-term studies also report increased fruit set after pollen supplementation, but not after resource addition. This pattern may reflect the fact that plants can store and reallocate resources, but not pollen, across multiple years. For animal-pollinated herbs such as these, uniting theories about pollination ecology and mast seeding may promote an understanding of the mechanisms that determine patterns of reproduction over time.